JP 2001-297748A discloses a nonaqueous electrolyte secondary battery comprising a flat battery element, a bag-like exterior case which houses the flat battery element, a positive electrode lead which is led out from a sealing part of the exterior case, and a negative electrode lead which is led out from the sealing part of the exterior case. The flat battery element is configured such that sheet-like or film-like positive electrode plates, separators for holding electrolyte, and negative electrode plates are laminated. One end of the positive electrode lead is connected to the positive electrode plates and one end of the negative electrode lead is connected to the negative electrode plates.
In the above secondary battery, however, the interior space of the battery formed between the power generation element and the case is not taken into account. Therefore, when the charge and discharge are repeated and accordingly the electrodes expand in the thickness direction to deform the case, the pressure applied from the laminate exterior case to the power generation element becomes ununiform to lead to ununiform cell reaction and poor durability of the cell.